Permeability is the capacity of a porous material, such as reservoir rock, to transmit fluids (the fluid conductivity of a rock). It is one of the most important parameters in determining the rate of fluid flow in a reservoir. Detailed permeability measurements on slabbed or unslabbed whole cores are especially useful for interpretation of geologically complex, heterogeneous formations.
The concept of using "probe permeameters" for non-destructive measurements of permeability is well known. One such device is made by Edinburgh Petroleum Services. These prior art probe permeameters utilize a steady-state technique.
A small diameter tube, or probe, is pushed against the surface of a core sample cut from the reservoir. The end of this probe is fitted with a rubber gasket or O-ring, which makes a gas-tight seal between the sample and the probe. Air or nitrogen is delivered from the probe through the interior of the seal to the rock sample at a fixed, measured pressure. The gas then diffuses through the rock sample, starting from the spot beneath the seal, diverging in a somewhat hemispherical pattern and ultimately passing from the upper surface of the sample just beyond the outer diameter of the gasket or O-ring, and from other surfaces of the sample that are exposed to atmospheric pressure.
The flowrate and pressure of the gas delivered from the probe is measured directly using devices such as a mass flow meter or rotameter and a pressure transducer or pressure gauge. When the flowrate and pressure become constant (invariant with time), "steady state" is achieved. The upstream, delivered pressure and flowrate are then recorded and the permeability is calculated. A variant of this prior art, steady-state procedure consists of holding the upstream, delivered flowrate constant by means of a flow controller. When the upstream, delivered pressure becomes invariant with time, steady-state conditions are attained and the measurements are made.
One disadvantage of the steady-state measurements is the long period of time that elapses before steady-state conditions are reached. The wait time to reach steady-state increases as the permeability of the core material decreases. It reaches twenty minutes or more for low permeability samples. Another drawback is the need for directly measuring the flowrate. Most flowrate measuring devices have a very limited range with accuracy only reaching about.+-.1%.
The range of permeabilities that can be measured by existing commercial probe permeameters is claimed to be from about 0.1 millidarcys (md) to about 10,000 md. Measurements of permeabilities below about 5 md, however, can have a large percentage error.